Most load cells ask you to build them into the load path. You interrupt the thing carrying force, insert the sensor, and reassemble around it. Threaded cells screw into the chain. Compression cells sit between two surfaces that press together. Either way, the sensor becomes a link in a chain that previously did not include it.
A through-hole design inverts that. Instead of interrupting the load path, it wraps around something already in it. Bore a hole through the middle of a compression element, slide it over a bolt, a stud, a shaft, or a tie rod, and the force already travelling through that component now passes through the sensor as well. Nothing gets cut, nothing gets rerouted, and the assembly works the way it did before with a measurement added to it.
That single geometric decision changes what the sensor is for. Bolt preload, clamping force in a stack-up, tension in a tie rod, thrust behind a bearing, the force a fastener actually sees rather than the torque you applied to it — these are measurements that are difficult to make any other way, because the thing you want to measure is already assembled and you would rather not take it apart.
Inside, it is a compression sensor like any other: strain gauges bonded to a machined element, a Wheatstone bridge, a millivolt output proportional to load. What differs is that a donut carries force through an annulus rather than a solid body, which makes the geometry work harder. The wall between the bore and the outside diameter is doing the sensing, so on a compact unit that annulus can be quite thin — which is why through-hole cells are machined from strong material such as heat-treated 17-4 PH stainless, and why how evenly load is introduced around the ring matters more than it would on a solid button.
You'll find them across bolted-joint verification, fastener and assembly force monitoring, motor and actuator bench testing, robotic end-effector force sensing, materials testing, aerospace structural work, biomechanics research, and any automation station where a fastener or shaft has to be confirmed rather than assumed.
The measuring chain is conventional. The installation is where through-hole sensors have their own rules.
Signal. Power the bridge — 10 VDC here — and it becomes ready to answer. Squeezing the element nudges each gauge's resistance a little, the bridge's symmetry breaks, and a handful of millivolts appears where previously there was almost nothing. That handful is proportional to what you supplied, hence sensitivity carrying volts on both sides of the slash. Amplification, filtering and conversion follow, the certificate does the translation, and force appears. Deviate from the supply voltage the certificate assumed and every number inherits that deviation.
The bore is clearance, not a bearing. Few things about these sensors get misread more often. The hole exists so the bolt or shaft can pass through unobstructed — it is not a load path and it is not a locating feature. The component through the middle should pass concentrically without touching the bore wall, because contact there introduces friction and side force that the sensing element cannot distinguish from the axial load you want. If your bolt is rubbing the bore under load, something in the alignment needs correcting before the readings mean anything.
Force has to enter evenly around the ring. A donut senses through the annulus between bore and outside diameter, so load arriving on one side of that ring rather than distributed around it produces a reading that depends on where the pressure happened to land. In practice this means the surfaces contacting each face — washers, nuts, flanges, whatever the assembly provides — need to be flat, hard, and large enough to bear across the full annular face rather than on part of it. A standard fastener washer is often too small or too soft for the job.
Preload and reading interact. Because these sensors are typically installed under a fastener that then gets tightened, the act of assembly applies load. That is usually the point — but it means the sequence matters. Zeroing after the joint is clamped removes the preload from your reading; zeroing before shows you the preload itself. Decide which of the two you actually want before you tighten anything.
Stiffness, temperature, direction. Low deflection means fast settling and good dynamic behaviour. Published temperature coefficients turn a known thermal swing into a calculable error rather than an unknown. And compression calibration describes compression only — tension travels a different load path and is a separate exercise.
Choosing a through-hole cell runs backwards compared with most sensors. Capacity is usually the second question, not the first.
Start with what has to pass through it. Measure the bolt, stud, shaft, or rod, and work from there. The bore must clear that component comfortably — enough room that it never contacts the wall under load or during assembly, without being so oversized that the sensor grows unnecessarily. Where a series offers more than one bore per capacity, that choice is genuinely a specification decision and not a detail; get it wrong and the sensor either will not fit or will read badly.
Then check the outside diameter against the space you have. The other constraint is what the sensor has to fit inside — a counterbore, a recess, the flat between a nut and a flange. Compact outside diameters open up installations where a standard donut simply will not go, and in retrofit work that is frequently the deciding factor rather than any electrical specification.
Only then size the capacity. Your day-to-day force should land comfortably within range so resolution stays useful, and your credible maximum should still fall under the safe overload figure. On bolted joints, remember that the force you can generate with a wrench and a lever is often far more than the force the joint is meant to see — overload during assembly is a real risk, not a theoretical one.
Plan how load reaches the faces. Hardened, flat bearing surfaces of adequate diameter on both faces, sized to cover the annular area rather than press on part of it. If the existing hardware in your assembly cannot do that, the right answer is usually a pair of properly sized hardened washers rather than accepting whatever the joint already has.
Confirm direction and environment. A compression unit's certificate covers pushing alone, so flag any pulling at the point of order. For temperature, the compensated band marks where the published numbers apply and the drift coefficients let you predict an error instead of discovering one.
And decide what reads it. A regulated supply, an amplifier signal conditioner module, or a digital display with alarms, analog output or logging — and if you are monitoring several joints, work out early whether you need each one read individually or a combined total, because that decides how many channels you are buying. Cal-Teds plug and play is worth having where sensors rotate between assemblies.
Send us the diameter of what passes through, the space available around it, and the force you expect, and we will tell you which configuration fits. Stocked items ship next day, and academic rates are available for teaching and research.
THA Series Load Cell Applications.
The Transducer Techniques THA Series through-hole donut load cells, characterized by their compact design and multiple through-hole diameter options, find applications in various industries and scenarios where precise force measurement is essential.
- Force and Torque Measurement: The THA Series load cells are frequently used in force and torque measurement applications, including bench testing of motors, engines, and various mechanical components.
- Automotive Testing: Automotive manufacturers and testing facilities utilize THA Series load cells for testing and quality control of various vehicle components, such as suspension systems, steering mechanisms, and brake systems.
- Robotics and Automation: THA Series load cells are integrated into robotic and automation systems for tasks that involve force sensing, such as robot end-effector force control, pick-and-place operations, and assembly line quality checks.
- Materials Testing: In materials testing laboratories, THA Series load cells are used for various applications, including tensile and compressive testing of materials such as metals, plastics, textiles, and rubber.
- Aerospace and Aircraft Testing: The aerospace industry relies on THA Series load cells for structural testing of aircraft components, flight control systems, and materials used in aircraft construction.
- Industrial Automation: THA Series load cells are integrated into industrial automation systems for monitoring and controlling forces in manufacturing processes, ensuring precise assembly and quality control.
- Product Development: Engineers and researchers use THA Series load cells during product development and prototyping to evaluate the performance and durability of new designs and components.
- Biomechanics and Medical Devices: In biomechanics research and medical device testing, THA Series load cells measure forces applied to the human body or medical devices during testing and analysis.
- Educational Laboratories: Educational institutions incorporate THA Series load cells into engineering and physics laboratories to teach students about force measurement principles and conduct experiments.
- Custom Machinery and Equipment: Manufacturers and research facilities integrate THA Series load cells into custom-built machinery and equipment designed for specialized testing and manufacturing processes.
Their multiple through-hole diameter options make them adaptable to various testing and monitoring scenarios.
Frequently Asked Questions
What is the difference between the THA -P and -Q models?
The bore diameter, and nothing else. The -P has a 0.128 inch through hole, nominally 1/8 inch; the -Q has a 0.193 inch through hole, nominally 3/16 inch. Both share the same 1.00 inch outside diameter and the same electrical specifications, so the suffix is telling you what will pass through the middle rather than anything about performance. Every capacity in the series is available in both, which means you choose bore and capacity independently.
What capacities does the THA Series offer?
Four: 50, 100, 250, and 500 lb. Combined with the two bore options that gives eight configurations, all at the same price. Because bore and capacity are independent choices, you don't have to accept a compromise on one to get what you need on the other — a 50 lb cell with the larger bore is just as available as a 500 lb cell with the smaller one.
Why does the THA's 1.00 inch outside diameter matter?
Because in through-hole work, the space available around the fastener is very often what decides whether a measurement is possible at all. At one inch across, the THA fits into counterbores, recesses, and gaps between nuts and flanges where a larger donut simply won't go — which makes it a practical option for retrofitting measurement into an assembly that was never designed with a sensor in mind.
How do I choose between the -P and -Q bore?
Measure what has to pass through, then pick the bore that clears it comfortably without being unnecessarily large. The component should never touch the bore wall during assembly or under load, so allow genuine clearance rather than a slip fit. If your bolt or shaft is close to 1/8 inch, the -P is the natural choice; approaching 3/16 inch, the -Q. If you fall awkwardly between the two, call us before ordering rather than forcing a fit.
What are the THA Series' accuracy specifications?
Nonlinearity and hysteresis are each 0.25% of rated output, nonrepeatability is 0.1% of rated output, and zero balance is 2.0% of rated output. That zero balance figure is higher than several of our other series, which is worth knowing but not alarming — it describes the residual offset with no load applied, and your instrumentation zeroes it out during setup. What it does mean is that zeroing properly, in the installed condition, is not a step to skip on this series.
What is the THA made from?
Heat-treated 17-4 PH stainless steel with bonded foil strain gauges. Material matters more than usual on a compact donut, because the sensing structure is the annular wall between the bore and the outside diameter — and at a 1.00 inch outside diameter with a bore through the middle, that wall isn't thick. 17-4 PH is a precipitation-hardening stainless whose heat treatment forms fine strengthening particles throughout the metal, giving the strength that geometry demands without sacrificing dimensional stability.
How much does a THA deflect under load?
0.002 inches at rated output. On a through-hole installation that figure carries a practical meaning beyond stiffness: the sensor is usually sitting inside a clamped joint, and how much it compresses affects the joint's own behaviour. Two thousandths of an inch is small enough that adding a THA to a bolted stack-up doesn't meaningfully change how that joint behaves once tightened.
Is the THA compression only?
It ships calibrated in compression, which suits how donut cells are normally used — captured in a joint and squeezed as the assembly is tightened. Tension calibration is available as an option. Worth thinking about at order time if your application pulls rather than clamps, since the load path differs between directions and a compression certificate describes compression alone.
How does temperature affect a THA reading?
Compensation runs from 60° to 160°F with a safe operating range of −65° to 200°F. Within the compensated band, output drifts 0.005% of load per °F and zero drifts 0.01% of rated output per °F. Bolted-joint measurements are often long-duration, so on a monitoring application it's worth doing that arithmetic against your expected temperature swing rather than treating any slow change in reading as a loosening joint.
Can I get the THA with the plug-and-play TEDS option?
Yes, OPT-TEDS is available. It carries the unit's calibration data at the connector so a compatible instrument configures itself rather than relying on manual entry. With eight configurations across the series that all look broadly similar once installed, it's a sensible way to make sure the right calibration is applied to the right sensor — particularly if you have several THAs in service at once.
Questions From The Field
My bolt diameter falls between the -P and -Q bores. What should I do?
Go up to the -Q rather than trying to make the -P work. A bore that's marginally too small either won't accept the bolt at all or will leave it touching the wall, and bore contact is the single most common way to corrupt a through-hole reading. The larger bore costs you nothing in capacity or accuracy, since both models share identical specifications. If neither bore suits your component, contact us — there may be another series in the range that fits better.
Does it matter if the bolt touches the inside of the through hole?
Yes, and it's worth being deliberate about avoiding it. The bore exists purely as clearance; it is not a bearing surface and not a locating feature. A bolt rubbing the bore wall introduces friction and side loading that the sensing element registers alongside the axial force you actually want, and neither you nor the sensor can separate the two afterward. If contact is happening, the fix is in the alignment of the assembly rather than anywhere in the instrumentation.
What should the washers on either side of my THA look like?
Hardened, flat, and large enough to bear across the full annular face rather than press on part of it. A standard fastener washer is frequently too small in diameter or too soft, and a soft washer indents under load so the contact area shifts as force builds. Since the sensing structure is the ring between bore and outside diameter, load arriving on only part of that ring gives you a reading that depends on where the pressure happened to land rather than on the actual force.
Should I zero the THA before tightening the bolt, or after?
Depends entirely on what you're trying to measure, and it's worth deciding before you pick up a wrench. Zero before tightening and the sensor shows you the clamping force the joint develops as you torque it — which is usually what people want when they're verifying preload. Zero after the joint is clamped and you've subtracted that preload, so the reading afterward shows only additional load applied on top of it. Both are legitimate; measuring the wrong one is a common and entirely avoidable mistake.
My reading drifts down slowly after I torque the joint. Is the sensor faulty?
Usually not — bolted joints genuinely relax after assembly as surfaces bed in and any coatings or soft components settle, and a through-hole cell reports that honestly. Give the joint time to settle and re-check, and compare the rate of change against the published temperature coefficients if the environment is also changing. Persistent, continuing loss of load points at the joint design rather than the instrumentation, which is precisely the kind of thing this sensor exists to reveal.
Can I fit a THA into an assembly that's already built?
That's much of the point of a through-hole design, and the 1.00 inch outside diameter helps. You'll need to back off the fastener, slide the cell over the bolt or stud with appropriate washers on each face, and reassemble — so you need enough grip length to accommodate the sensor's thickness plus washers. Check that available length before ordering, along with whether the surrounding structure leaves room for a one-inch diameter body, since those two dimensions decide feasibility more often than the electrical specification does.
I'm monitoring several bolts. Do I need a THA on every one?
Depends on what you need to know. If any individual fastener losing preload would matter, then yes — a cell per bolt is the only way to see which one moved. If you only need confidence that total clamping force across the joint is holding, instrumenting a representative subset is often adequate and considerably cheaper. Tell us the joint and what failure would look like, and we can help you decide how many channels the job actually justifies.
Can I move a THA between different assemblies?
Yes, provided each installation gives it a proper seat — hardened flat bearing surfaces on both faces and genuine clearance around the bore. Re-zero after every installation as routine, because the clamping condition won't reproduce exactly from one assembly to the next and the resulting small offset is normal rather than a fault. If sensors move regularly, the OPT-TEDS option keeps each unit's calibration travelling with it instead of depending on someone remembering which cell is fitted where.